An hydrated Potassium,Calcium sulfate with formula K2Ca(SO4)2-(H2O) and hardness 2.5.
Typical evaporite saline mineral or volcanic sublimate; soluble in water.
Rarely transparent, often included with lots of impurity. This gem here comes from Kalush ( also called Kalushite) deposit,Ucraina.
This gem here is the best we can get.